Problem is that boost mana could very quickly become a gnome-specific boon if it gets nerfed. With boost health, the numbers are quite low so it's mostly used to survive blows that would otherwise be 1HKO's, a situation where health potions don't do much good anyways (besides the fact, simply chugging the potion for +3 attack from a Trisword and saving the piety for something else is probably a better bet for a Halfling).

Honestly I don't like either boon flavor wise.. doesn't feel very chaotic, in fact it sort of promotes personal control. Make them cause potions to give randomly between 20-60% or if you want it to on average be a bug, be 30-70%. Have it increase the ranges by 10% for every potion you convert.

So 2 converted potions will always be at least as good as a normal potion and sometimes up to twice as good. You gain strength at the cost of comfort!

Any time you're increasing the value of future potions at the expense of existing potions, you're going to run into the same problem that you have with gnomes right now - unless the boon is not strong enough to break gnomes, in which case no one else will want it at all.